feat: make DDNS domain_name the effective domain across all services

- ConfigManager.get_effective_domain(): returns domain_name when DDNS
  active (pic_ngo/cloudflare/duckdns), domain otherwise. Used by all
  public-facing services so they use the real registered FQDN.
- ConfigManager.get_internal_domain(): always returns _identity.domain
  (CoreDNS zone name, dnsmasq, cell-link invites — stays internal).
- Silent migration: if domain_mode != lan and domain is generic "cell",
  auto-set to {cell_name}.local for unique CoreDNS zone naming.
- caddy_manager: fix custom_domain bug — cloudflare/http01 modes were
  reading identity.get('custom_domain') which never exists; now reads
  domain_name correctly.
- routes/config, app: expose effective_domain in GET /api/config and
  /api/status responses.
- email_manager, routes/email: use get_effective_domain() for
  OVERRIDE_HOSTNAME, POSTMASTER_ADDRESS, and new-user email defaults.
- ServiceBus.IDENTITY_CHANGED event: emitted from PUT /api/config and
  POST /api/ddns/register after identity writes; caddy_manager and
  email_manager subscribe to regenerate config automatically.
- Settings.jsx: hide Local Domain input in non-LAN modes; show
  read-only effective_domain with "managed by DDNS" badge and an
  Advanced toggle for the internal CoreDNS zone name.
- 11 new test classes covering all new helpers, event subscriptions,
  caddy/email handlers, and the custom_domain fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/config_manager.py

@@ -45,6 +45,21 @@ class ConfigManager:
             self.configs['connectivity'] = {'exits': {}, 'peer_exit_map': {}}
         if not self.config_file.exists():
             self._save_all_configs()
+        # Silent migration: when DDNS is active but the internal domain is still
+        # the generic "cell" default, give CoreDNS a unique zone name so multiple
+        # cells on the same LAN don't collide.
+        try:
+            _ident = self.configs.get('_identity', {})
+            _mode = _ident.get('domain_mode', 'lan')
+            _domain = _ident.get('domain', '')
+            _cell_name = _ident.get('cell_name', '')
+            if (_mode != 'lan' and _cell_name
+                    and (_domain in ('cell', '', None))):
+                _new_domain = f'{_cell_name}.local'
+                self.configs['_identity']['domain'] = _new_domain
+                self._save_all_configs()
+        except Exception:
+            pass
 
     def _load_service_schemas(self) -> Dict[str, Dict]:
         """Load configuration schemas for all services"""
@@ -478,6 +493,23 @@ class ConfigManager:
         """Return the current identity configuration."""
         return self.configs.get('_identity', {})
 
+    def get_effective_domain(self) -> str:
+        """Return the FQDN that public-facing services should use.
+        In lan mode: _identity.domain. Otherwise: _identity.domain_name
+        (falls back to domain if domain_name not yet registered)."""
+        ident = self.get_identity()
+        mode = ident.get('domain_mode', 'lan')
+        if mode == 'lan':
+            return ident.get('domain') or os.environ.get('CELL_DOMAIN', 'cell')
+        return (ident.get('domain_name')
+                or ident.get('domain')
+                or os.environ.get('CELL_DOMAIN', 'cell'))
+
+    def get_internal_domain(self) -> str:
+        """Return the CoreDNS zone name (always _identity.domain)."""
+        ident = self.get_identity()
+        return ident.get('domain') or os.environ.get('CELL_DOMAIN', 'cell')
+
     def set_identity_field(self, key: str, value: Any):
         """Set a single field in the identity configuration and persist."""
         if '_identity' not in self.configs: